The co-pending application of Louis W. Johnson, entitled "Improved Vibrating Screen Apparatus", Ser. No. 638,535, filed Dec. 8, 1975, now abandoned, discloses a vibrating screen apparatus with plural unbalanced shaft assemblies. Each shaft assembly includes a gear hub and an annular gear. In at least one of these shaft assemblies the gear hub and its shaft can be turned relative to the annular gear when a locking mechanism between the annular gear and the gear hub is released, to facilitate varying the angle of stroke imparted to the vibrating screen. In the previous construction, the locking mechanism comprised a clamp at the mating edges of the gear hub and the annular gear. When the annular gear is made relatively thin (to save metal) this clamping arrangement tends to bow it outwardly making for operating difficulties.